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Brown spider monkeys have long and thin limbs with their forelimbs being longer than their hind limbs. They also have a distinctive 75 cm (30 in) long flexible, thin and prehensile tail which at times acts like a fifth limb. The tail has a highly flexible, hairless tip with skin grooves which improves grip. Their hands look slightly curvy and they have small thumbs. All these features make it possible for them to climb trees at high elevations, hang and swing from one tree to another without often having to lower themselves to the ground. Adult males weigh between 7.9 and 9.1 kg (17 and 20 lb) and adult females weigh between 7.5 and 9 kg (17 and 20 lb). Their average adult body length is about 50 cm (20 in). Their coloration ranges from light brown to dark on upper parts including the head. Their most distinctive characteristic is a whitish triangular forehead patch, although not all spider monkeys have one. Some few individuals have pale blue eyes.[4]

Two subspecies are often recognized: A. h. hybridus (in both Colombia and Venezuela) and A. h. brunneus (between Cauca and Magdalena River in Colombia),[2] but these are sometimes considered synonymous.[1]

Even though brown spider monkeys spend most of their time high in trees, they occasionally descend to eat soil and drink water. Since they like to forage in high canopies, they prefer undisturbed primary forests. This is because primary forests offer lifestyle they are comfortable in, for example endless canopies, tall trees, and many fruiting trees that disturbed forests do not offer.[6] Brown spider monkeys always travel in small groups, and instead of walking or running on all fours, they travel mostly by swinging and climbing between trees.[2]

Brown spider monkeys mainly forage in the canopies of forests and rely mostly on their senses of sight, smell, taste, and touch to find food. Brown spider monkeys are mainly herbivores and frugivores. A main component of the brown spider monkey’s diet is ripe fruit. 83% of their diet is lipid rich fruits. However, in drier seasons where fruit is less abundant, brown spider monkeys feed on leaves, seeds, flowers, bark, honey, decaying wood, and occasionally insects such as termites and caterpillars.[2] Brown spider monkeys feed on different species of figs year around. Scientists have observed spider monkeys eating soil and clay, and hypothesized that the reasons for this behavior could be to obtain minerals from the soil, for phosphorus, or in order to maintain a pH-balance in their digestive system. Brown spider monkeys find water to drink on the forest floor at “salado sites.” Competition for food occurs between spider monkeys and other frugivorous primates.[7]

Average lifespan of a spider monkey is 27 years, however, in captivity that can be increased by 10 or more years.[8]

The population is estimated to have decreased by at least 80% and some populations have already been extirpated.[10] Few remaining populations are of adequate size to be viable long-term.[2] Almost 60 brown spider monkeys were recorded at various zoo (mostly European) that participated in the International Species Information System in 2010, but breeding is slow.[11]Habitat loss is ongoing within its wild range, and an estimated 98% of its habitat already is gone.[12] Habitat loss is driven both by logging, and land clearance for agriculture and cattle ranches.[2] It is also threatened by hunting (in some regions it is the favorite game) and the wild animals trade.[10] One study did not show a significant difference between population densities inside versus outside forest areas disturbed by loggers. It has been hypothesized that this anomaly is due to the sample being taken from El Paujil reserve, which is a protected area and may serve as refuge from other human activities, namely poaching.[13]

A small population of fewer than 30 individuals of the subspecies A. h. brunneus has been discovered in a protected area of Colombia, the Selva de Florencia National Park. This is the southernmost population of the brown spider monkey.[14] Brown spider monkeys are also known from other reserves in both Colombia and Venezuela.[2]